texts:
Refer to the below minutephysics video, in which they say that in order to reach the Sun from earth, we must first stop revolving around the Sun ie make our tangential velocity zero, and then head straight for the sun, otherwise it is nearly impossible to reach the sun because even a very huge push towards the sun will either send us in an elliptical or hyperbolic orbit:-

https://www.youtube.com/watch?v=LHvR1fRTW8g

If even a little bit of orbital speed tangentially, prevents something from falling into the sun, then I wonder how does the sun formed itself at the beginning? Because, most collapsing material probably would be having at least some tangential velocity with respect to the center.


texts: How would astronomical seeing on Mars differ from that on Earth?

texts:
Astronomical seeing is the limiting factor for the resolution of all but the smallest Earthbound telescopes.

 Source

Stunning advances in adaptive optics (along with it's predecessor speckle interferometry and it's budget-minded cousin lucky imaging) get around this but only with substantial compromises in (some combination of) throughput, cost, complexity and wavelength range (see answers to Why aren't ground-based observatories using adaptive optics for visible wavelengths?)

If identical telescopes sat on the surface of Earth and Mars and looked at a distant and equi-distant  body, how would the following differ between the Mars telescope and the Earth telescope?:


texts: What is the brightest star (by absolute magnitude) that we can see by naked eye?

texts:
Is it Rho Cassiopeiae or Mu Sagittarii?
I see in the Stellarium 0.13.0 that Mu Sagittarii has absolute magnitude -11.43!!


texts: Let's talk about the time when our sun becomes a supergiant. We know that our solar system may get destroyed or it may get affected due the large mass of the supergiant and also due to the large temperature. But somehow let us consider nothing happened to our solar system by luck. It's obvious that life won't be possible on earth anymore(due to the extreme heat), but is there a possibility that, the events that took place during the formation of Earth, may take place in the far off planets? Can life formation take place in some other planet just like it happened in Earth??


texts: I am told in a lecture that some particular meteorite is from Vesta. How can we know that a particular meteorite is from there?


texts: Why are the natural satellites (moons) of all planets solid?

texts: Why are the natural satelites (moons) of all planets - including the moons of the gas giants - solid or rocky, and not gaseous?


texts:
I always hear the narrator of documentaries say that a star exploded because it ran out of fuel.
Usually things explode when they have too much fuel, not when they run out of fuel.
Please explain...


texts:
Consider a loose pile of rocks in an early solar system, say a tiny asteroid. As it accretes more and more material, the pressure at its center eventually becomes great enough to collapse the gaps between the rocks, which further increases the density, which increases the gravitational force, ... which leads to a runaway process that compacts a loose pile of rubble into a near-spherical body of almost solid rock.  (And some ices and liquids.)

But where do the rocks themselves come from?  Surely at least that much pressure is needed to fuse dust into macroscopic rocks?  Why isn't this a chicken-and-egg problem?


texts:
After reading this answer I wondered if there have been document comets with orbits on non-return, hyperbolic trajectories as they approached the Sun.

This means that they would not be gravitationally bound to the sun, so either they came from outside the solar system, or they managed to gain energy through a collision, or close pass with a large planet like Jupiter.

The quickest way to find the comet with the "highest" orbit is to ask for the one with the largest specific orbital energy, the total energy (kinetic + potential) divided by the mass, (Joules/kg). For a bound orbit the value would be negative, for an unbound, non-returning orbit it would be positive.

Question: What documented comet has/had the highest specific orbital energy? It should be an identified object with a known or measured orbit, not just a generic, nameless "edge-of-the-Oort Cloud placeholder."


texts:
I have some basic understanding of the Kepler/TESS mission. The missions both look at the stars and try to find the dip in the light from the stars to find possible existence of exoplanets (exomoons as well). I was trying to find the dataset. I found one URL to retrieve the datasets. The URL is given below.

http://exoplanetarchive.ipac.caltech.edu:80/data/ETSS//Kepler/005/141/63/kplr010666592-2009131105131_llc.fits

But this URL contains the filename. But there should be several other files as well. To find the other files, I wanted to check the above URL without the filename (http://exoplanetarchive.ipac.caltech.edu:80/data/ETSS//Kepler/005/141/63/). But this URL shows error : "Forbidden
You don't have permission to access /data/ETSS//Kepler/005/141/63/ on this server."

I even created an user account in https://exoplanetarchive.ipac.caltech.edu/ but still I am receiving the same error. Please assist me, how I can get the keplar data.

Also I found that the file mentioned above is very small only a few KB and after converting it to csv I found that it contains only less 2000 rows. But I know that it needs several hours of observation to find out that an exoplanet, that means the dataset cannot be such a small dataset. Please explain why the dataset is so small. Is it only a part of the dataset, in that case please let me know how to get the full dataset.


texts: Could there really be a preferential direction to the speed of light?

texts:
This question is inspired by a recent video by Veritasium Why no one has measured the speed of light linked below.
To summarize the video, Derek points out that it is impossible to measure the one-way speed of light, and shows examples of what the consequences of a preferential direction for the speed of light might be.
While the examples given of the measurement problem and consequences all appear valid on local scales, they seem to me to fall apart when you look at the scale of the observable universe. Wouldn't we be able to observe the effects of a preferential direction to the speed of light at cosmological scales? Take the extreme case: the speed of light in one direction is 1/2c and infinite in the opposite direction. If this was true, we should be able to observe the entire universe in the direction where light is approaching us at infinite speed, and see no redshift due to the expansion of the universe (because the photons would reach us instantaneously, there would be no time for the expansion of the universe to stretch them out). Even in less extreme cases, we should still see "more" universe and less redshift in one direction than its opposite.
Since we don't (to my knowledge) observe any difference in the "quantity" of universe (for lack of a better term, feel free to edit if there is a better term for this) in any direction, or any difference in the amount of redshift in any direction, then if there exists a preferential direction to the speed of light, it must be small enough that its effects lie within the error bars for our ability to measure the universe on cosmic scales.
Is my reasoning correct here, or is there some effect I did not take into account that would adjust things so we wouldn't see any difference? (or perhaps my understanding of cosmology is entirely flawed?)


texts: Cosmological redshift and comparing past galaxy sizes

texts:
If you were to divide the present-day
universe into cubes with sides 10 million light-years long, each
cube would contain, on average, about one galaxy similar in size
to the Milky Way. Now suppose you travel back in time, to an era
when the average distance between galaxies is one quarter of its
current value, corresponding to a cosmological redshift of z = 3.
How many galaxies similar in size to the Milky Way would you
expect to find, on average, in cubes of that same size? In order to
simplify the problem, assume that the total number of galaxies of
each type has not changed between then and now. Based on your
answer, would you expect collisions to be much more frequent at
that time or only moderately more frequent?

I am very confused with this question. From my intuition, I take that volume would equal to distance^3. They have given us that the past distance would be 1/4 of current. Therefore, do I cube 1/4distance? Also how does the redshift of 3 play a part in the solution. Thanks for any help! I do not need an answer, just how to start.


texts: Plotting GAIA barycentric radial velocity data with python

texts:
Does anybody know of a good tutorial on how to use python and astropy to generate plots of GAIA, RAVE or any other database of barycentric (heliocentric) radial velocity data of stars in our galaxy?

I'd like to generate various plots of the heliocentric velocities of stars in our galaxy, making different restrictions on distances from the sun, and I'd like to see what the data looks like with correction for the proper motion of the sun and without. Now, I know I could find plots done by others, but I really want to learn how to generate my own plots.

I'm pretty new to python, so please target answers accordingly...

(added later:) There's probably a simple astropy function that if you call it with the right variables, say, a certain slice of the sky in RA and Dec, and a certain distance range in kpc, it will spit back a list of all the stars in that region along with their radial velocities.  I just need to learn the name of that function and how to call it.


texts: Milky Way mass fraction breakdown review

texts:
I'd like to know a breakdown of the mass of the Milky Way, by the major categories. So, something like

Stars       ... 3%
Cool nebula ... 7%
Gray matter ... 12%
Etc
Etc

This order-of-magnitude knowledge that everyone should know.

(Of course, when talking of the galaxy, we can, obviously, think of either just the visible disk-bulge, or perhaps, a greater unit of the galaxy sitting in a "bulb" (of - perhaps - dark matter (?), gas (?), dust (?), halo stars (?) or something else): a brief clear answer in a few sentences would expose these concepts.)

Surprisingly, this information is nowhere to be found in one place as an overview.

I appreciate that the largest chunk may be dark matter (is that right?); but I don't even have a ballpark idea for the rest. It's the sort of knowledge that is obvious to professionals, but surprisingly not outlined in any of the usual popular science sources.


texts: A rough analytical expression for the Milky Way's radial mass distribution?

texts:
I found the image below in Space.com's article This 3D Color Map of 1.7 Billion Stars in the Milky Way Is the Best Ever Made, although it is not the map mentioned in the title.

The caption for this image reads:

  This radial velocity image shows the movement of 7 billion stars. The colors run from blue (stars moving at 50 km/s toward us) to red (stars moving 50 km/s away from us). The white color shows when, on average, the stars are not moving in the line of sight with respect to us. Stars lagging behind as they orbit the center of the Milky Way appear to be traveling away from us, and those speeding up appear to be traveling toward us. Credit: ESA/Gaia/DPAC

If you imagine a band along the galactic equator the dominant velocity shows two positive and two negative "peaks", with a zero crossing in the direction of the galactic center.

Purely for fun I wanted to see if I could reproduce this behavior with a simple calculation based on a 2D calculation assuming circular motion and a radial density distribution $\rho(r)$ which I could then use to figure out a rotational velocity distribution $v(r)$, bun I swiftly realized that I have no idea what the density profile would look like.

For the purposes of this simple exercise, what would be an analytical expression that roughly matches the Milky Way's radial density profile, projected on to its equatorial plane?
For spherically symmetric distributions, Newton's Shell theorem allows one to treat all mass inside a sphere defined by an orbit's radius as if it were at the center, and to ignore all mass in the shell outside of that radius. Is there anything like an analog to this for a radial distribution within a plane?


texts: Is there an all sky meteor watch with spectroscopy?

texts:
I assume performing spectroscopy on meteors would give us information about the composition of the outer layers of meteoroids, which we can't study on finds because it has already burned up. This would increase our knowledge on the small bodies of the solar system.
There are programs, like CAMS or FRIPON, that watch the entire sky to spot meteors. These are just regular cameras.
We can perform spectroscopy for many points in the sky simultaneously, like with MUSE, so it would be possible to monitor most of the sky continuously to get spectroscopic data of meteors.
Has this been done? If not, why not?


texts: Orbits of planets in ecliptic coordinates

texts: Is it possible to get the ecliptic coordinates of orbits of planets for a specific date and time? Does the ephemeris data of planets help in constructing that? Any help on this would be helpful. I would like to get the planetary orbit data that I can plot in a map.


texts: What is the physical, geometric shape of the universe?

texts:
I'm not asking about theoretical ball, vs saddle, vs flat surface which is just a metaphor with 2D space.

It's hard to say as we see very little of it, and we see them in the past because light travels for so long. But what we do know is that it is inflating (not exploding as one might thing from the "big bang" naming).

How would the universe look like, if we were to freeze it in a moment, is it likely to be a ball, or rugby ball, a cone or a sort of an irregular shape?

Is it filled throughout with galaxies, dust, black-holes, or does it live on the edges of its 3D shape, and the central part is "empty"?

Does it have a giant black hole or a star in the middle around which everything revolves?


texts:
After folding a light curve, we may  get some kind of shape.
How to tell whether there is a real periodical signal quantifiably?

The fluctuation may be small, and the folded shape may not be perfect.


texts: How precise are the observational measurements for the speed of gravity?

texts: General Relativity says that gravity moves at the speed of c.  How precise are the measurements?


texts: Could rays of light be ever trapped in a constant orbit around a black hole if approached from a certain angle ? Like light hitting a glass or liquid with critical angle.


texts:
I read in many astronomy sites that Sirius is the brightest star of the night sky and that Orion’s Belt points to Sirius. For some days I have been regularly watching Orion (just before dawn) but the star that the Orion's belt points towards is of very average brightness; I don't think it is Sirius. On the other hand I found a very bright star near Orion , albeit not along the line of its belt. On the morning of October 9 (around 5:30 am) this star was adjacent to the crescent moon and shining slightly more brightly ! Even in the early morning glow when all the stars faded away it could still be seen along with the moon. Is this Sirius?

I live in Delhi so I am talking of the Delhi night sky.


texts:
I need to find the two times of zenith passage of the Galactic Equator for a given Earth Lat/Lon on any day between 3500 BC and 1500 AD.  Since the Galactic Equator is a line and not a point, I try to use PyEphem to create a zenith point for my geographic location (see code below), then find the Galactic coordinates of that point.  When the Galactic latitude of this zenith point is zero, that's the time of zenith passage of the Galactic Equator.

However, I think I need my zenith point to be in Equatorial coordinates (not Ecliptic) to accomodate the geographic location and the date/time, but I can't figure out how to make that work.

Here is my Python script based on PyEphem:

import ephem
from math import *

D = (input("Enter date (yyyy/mm/dd): "))
d = ephem.Date(D)
print(d)

location = ephem.Observer()
location.lon = '-87.0'
location.lat = '20.0'
location.elevation = 12
location.pressure = 1013.0
import csv
my_list = []
with open(r'C:\Users\Mark\Desktop\PYTHON\gzt.csv', 'w', newline='') as f:
        for T in range(1,1440):
                dm = ephem.Date(d + ephem.minute*T)
                location.date = dm
                ra, dec = location.radec_of('0', '90')
                ec = ephem.Ecliptic(ra, dec, epoch = d)
                eq = ephem.Equatorial(ec)
                #print(eq.ra, eq.dec)
                z = ephem.FixedBody()
                z._ra = eq.ra
                z._dec =eq.dec
                #z._epoch = d
                z.compute()
                #print(z.ra, z.dec)
                zg = ephem.Galactic(z.a_ra, z.a_dec)
                dmt = (('%s') % (dm))
                #print(dmt, 'Zlat = ', zg.lat)
                AZ = (('%s  %s') % (dmt, zg.lat))
                my_list = [AZ]
                writer = csv.writer(f, dialect='excel-tab')
                writer.writerow(my_list)


texts: Why is the Nancy Grace Roman Space Telescope (WFIRST) coronagraph considered "beyond-state-of-the-art"?

texts:
After about 01:30 in the NASA video NASA's Nancy Grace Roman Space Telescope: Broadening Our Cosmic Horizons the narrator says:

  To deepen its study of exoplanets the Roman Space Telescope will house a beyond-state-of-the-art chronograph, that will directly image and analyze Neptune-size planets in orbits slightly larger than Earth’s.

The video shows a diagonal mirror with an articulated phase shifting surface followed by some kind of what I am guessing is a pupil filter.

Questions:

What is this particular coronagraph or its design called, and where can I read in detail about how it works? Please feel free to add a short summary in the answer if so inclined.
What is it about the design that would be considered "beyond-state-of-the-art"?


texts: Is the SETI's `needle-sharp at 982.002 megahertz` signal accessible?

texts: In April-May 2019, the Parkes radio telescope, in Australia, received a weird signal while surveying Proxima Centauri. Is this signal available, ideally in some digitalized form? Where could I download it?


texts: Retrograde Motion: Stars in the background

texts:
I was reading about the retrograde motion of Mars. All explanations give reference to the stars in the background. But I do not understand why this is necessary. I say this since stars themselves also appear to move and can see and observe this without any references. This brings up the second point: if the stars also appear to move, why do explanations have these background stars frozen?
I am new to all this so this might be a naive question :)


texts:
So, I've been starting to investigate radio astronomy, and am wondering about if certain things are possible from an amateur standpoint. I was looking at this powerpoint(that discusses building a tiny radio telescope), http://nightsky.jpl.nasa.gov/download-redirect.cfm?Doc_ID=347&Doc_Filename=IBTcom1%2Eppt .

On Slides 11, 12, and 13, it shows various radio emissions overlayed on the orion constellation. As well as Slide 17 and 18.

I'm wondering, is the data acquired in these images, doable from an amateur standpoint? From previous research I think I can build some detection devices for the Hydrogen Line emission(1420Mhz/21cm line), and that could lead to data(per my initial/basic understanding)  that's shown in image 1. Am I right in that assumption, and are the other emissions or data doable from an amateur standpoint?


texts: How is the bar formed in a Barred-Spiral galaxy?

texts: How does the bar in a barred spiral galaxy form? What prevents it from being spirals all the way down like water spiralling down a plughole?


texts: How to understand this multi-day exposure photograph of Jupiter and the Moon

texts:
This answer includes a stunning photo of the Moon and Jupiter "Trails of the Shifting Moon" by photographer Jin Lu.

The caption reads:

  From the photographer: "I spent 4 days capturing the trail of the moon from the balcony of my house. This is a composite of 90 pictures, showing the shift of the path of the waxing Moon from crescent phase to near first quarter. Random floating clouds decorated the trails. The interval between each day's sequence of pictures ranges from 1.5 to 2 min. Besides the Moon, the most obvious celestial object in the composite is Jupiter. The trail of Jupiter is from pictures of Jan 9, the day the moon appeared closest to Jupiter in the evening sky. Although Jupiter looks closer to the Moon on Jan 8, that is not how it actually appeared then." - Jin Lu

I'm confused by two things:

The Moon is shown for January 7, 8, 9, 10 and the caption says "The trail of Jupiter is from pictures of Jan 9". I'd expect Jupiter to have contributions from all four days, not just one day. How can that be?
"Although Jupiter looks closer to the Moon on Jan 8, that is not how it actually appeared." If I measure the distance in pixels from the top-left Jupiter to the top-left Jan 8 and 9 Moons, the Jan 8 Moon is closer. How could that not be true in reality then?


texts: How to plot curvilinear coordinates in the frame of a Cartesian telescope field of view

texts:
I am wondering what projection is used to produce plots like this:

(Image taken from here, author: NASA/Ames/JPL-Caltech, Image credit: Software Bisque)

More generally, I am trying to figure out the telescope pointings (in az/el or an earth-centered-earth-fixed coordinate system preferably, RA/dec if the former are not possible or easy) for a tiled array of square images positioned next to each other, similar to how the Kepler CCDs are laid out.  I'm thinking that if I could know what projection is used to produce that plot, then I could step in x/y to wherever I want the images to be and then de-project to convert to RA/dec.

I have tried a gnomonic projection but I found that, with increasing distance from the survey center-point, more and more overlap occur between adjacent frames.  Things looked really good close to the center-point though.  I realized that a gnomonic projection isn't actually providing what I want, since I can move across the projection area a large amount (infinity in the limiting case) and still end up with az/el pointings that are on the horizon, despite moving over an infinite number of frames.


texts: How can I make the fringing pattern visible in my flat (twilight) images for filter i?

texts:
I have a bunch of twilight flats that have been preprocessed (dark, bias, overscan etc.) and need to remove the additive fringing from the (multiplicative) twilights. I normalized each one by dividing by the mean and then combined them using ccdproc in astropy. I then divided the normalized flats by this "new" flat and viewed them in DS9 but I still cannot see the fringing. I know it's there because I've seen it clearly before. I could use some helpful tips. TIA

PS I also tried in old skool Iraf and don't see anything obvious.


texts: What observatories provide data similar to USNO?

texts:
USNO (the United States Naval Observatory) generates positions for the celestial bodies (planets, moons,stars) and provides them in the form of .bsp files and their Astronomical Almanac.
Is there any alternative open publication produced by EU or other big countries?


texts: Find the height of the shooting star from the Earth's ground

texts:
One night, you and your friend were talking to each other. Your friend far about 20 km. from you. You told your friend that you see the shooting star pass through the sky at altitude of 75 degrees. Your friend also see this shooting star, but has difference apparent location by 6 degrees. Find the location of this shooting star, what is it hight from earth ground.

I think 6 degrees is the different of azimuth angle. But as I live in different lattitude, it may be effect the distance between friend and me. Or the answer is just 75degrees turn to the height from the earth ground. But how? Which and how does the correct way to solve this question?


texts: Estimating a star's radius, temperature, and luminosity based on its mass

texts:
(See updated figure and description below.)

I've been trying to generate ballpark estimates for the radius, temperature and luminosity of stars in the main sequence based solely on their masses (assuming the same composition for all stars).  The idea is to iterate through masses in steps of, say, 0.1 solar masses from 0.1 to 100, and roughly trace out the curve of the main sequence on an HR diagram.

For luminosity, I'm using something like this:

  if ( mass < 0.43 ) {
    L = 0.23*L_sun*pow(mass/M_sun,2.3);
  } else if ( mass < 2 ) {
    L = L_sun*pow(mass/M_sun,4);
  } else if ( mass < 55 ) {
    L = 1.4*L_sun*pow(mass/M_sun,3.5);
  } else {
    L = 32000*L_sun*mass/M_sun;
  }

For radius, I'm using something like this:

  if ( mass < 1 ) {
    R = R_sun*pow(mass/M_sun,0.57);
  } else {
    R = R_sun*pow(mass/M_sun,0.78);
  }

And, using these, I'm calculating the temperature from the luminosity-radius-temperature relation like this:

T = pow(L/(4.0*M_PI*R*R*sigma), 0.25);

where sigma is just the Stefan-Boltzmann constant.

All of this works about as well as I'd expect for stars of 1 solar mass or above, but breaks down completely for M dwarfs, as you can see from the attached image which shows my meager effort superimposed on HYG data.

Note that I'm plotting B-V here, calculated from temperature like this:

BV = pow(5601.0/T,1.5) - 0.4;

and absolute magnitude, calculated from luminosity like this:

magnitude = -2.5*log(L)/log(10) + 71.1974;

What can I do to improve this a little?  I'll add that this is aimed at being part of an exercise for undergraduates who are beginning programmers, so I'm looking for simple ballpark approximations, not anything fancy.

Update:
Based on Rob Jeffries' recommendation below, I took a look at the Mamajek data.  Here's a plot of B-V versus temperature from that data:

I've superimposed a fitting function of the form:

bv(t) = a/(b*t) - c

where the best-fit parameters are:

a = 4.2413
b = 0.000576479
c = 0.607144

Using this function to calculate B-V moves my numbers in the right direction, but it still doesn't quite do the trick, as shown in the figure below (the new B-V values are the curve that's lower on the right-hand side):


texts: What Causes the Large Radiation Fields Around Jupiter?

texts:
I had heard that the large radiation belts around Jupiter may be formed by liquid metallic hydrogen in (or around) Jupiter's core (which Wikipedia says haven't been observed in labs yet due to the immense pressure required), but I don't understand how the liquid metallic hydrogen can form a planetary magnetic field. The article also says that the hydrogen can act as a conductor, is this somehow related to moving electric fields forming magnetic fields?

Source: Magnetosphere of Jupiter - Wikipedia


texts: Converting from $\mathrm{W \: m^{-2}}$ to $\mathrm{Jy \: km \: s^{-1}}$

texts:
Suppose I observe a spectral line with rest frequency $\nu_0$ and integrated flux density $F = 10^{-26} \mathrm{W/m^2}$. Is the following line of reasoning correct?
$1 \: \mathrm{Jy} = 10^{-26} \: \mathrm{W/m^2/Hz}$ therefore $F = 1 \: \mathrm{Jy \: Hz}$. This can be interpreted as a peak flux in $\mathrm{Jy}$ multiplied by a linewidth in $\mathrm{Hz}$. Thus to convert to $\mathrm{Jy \: km \: s^{-1}}$ I just need to multiply the linewidth by $c/\nu_0$ to convert it from $\mathrm{Hz}$ to $\mathrm{km \: s^{-1}}$. Hence, $F = c/\nu_0 \: \mathrm{Jy \: km \: s^{-1}}$ (where $c$ is in units of $\mathrm{km \: s^{-1}}$).


texts:
I understand why hydrogen and helium are the most common elements in the universe. I also understand iron, because that's where stellar fusion stops. But why are (specifically) oxygen, carbon and neon more common than iron? I would expect iron to either outrank them, or some steady flow of atomic nnumbers to lead up to iron. These three, and the scattershot of seemingly random elements trailing behind iron, seem a very odd mix!

Source: https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements


texts: Are brown dwarfs which don't sustain any fusion considered stars?

texts:
I was reading about the coolest stars, and was surprised to find about stars with a surface temperature lower than a candle, like 2MASS 0939-2448 A/B, of about 100 Celsius like CFBDSIR 1458+10B and even below 0 Celsius temperature where the water freezes like WISE J0855-0714. Then I thought, they might be stars where fusion only happens very deep in the core, and then I found out this:
Brown dwarf

In addition, many brown dwarfs undergo no fusion; those at the low end
of the mass range (under 13 MJ) are never hot enough to fuse even
deuterium, and even those at the high end of the mass range (over 60
MJ) cool quickly enough that after 10 million years they no longer
undergo fusion.

Are these brown dwarfs which don't sustain any type of fusion considered stars? If so, what's the reason?


texts: How was the first exoplanet discovered? What method was used to detect it?


texts: how does redshift prove expansion is accelerating?

texts:
My astronomy teachers never would answer this for me... Redshift obviously indicates an object (such as another galaxy) is moving away, but how do we know its acceleration from this? It's my understanding that the instantaneous redshift of an object is an indicator solely of its instantaneous velocity, and we have certainly not been observing redshift long enough to compare and determine a change in velocity

If you can, please avoid using complex math in your answer.


texts: Did Eudoxus describe the eclipitic on his sun motion model?

texts:
In the description of Eudoxus’s model for the sun he says one the three circles which describe solar motion is “the circle which runs along the middle of the zodiac”. Does this refer to the ecliptic plane?
What I thought is that it does mean the same thing as the ecliptic, which as seen from earth when looking up to the night sky, appears as if it cuts the zodiac in the middle. In a spherical model, the Sun’s apparent movement can be described as running along the constellations of the zodiac throughout the year on a “circle” which is known as the ecliptic.

Am I getting this wrong?


texts: Latitude and longitude of Eugene Shoemaker's final resting place?

texts: Eugene Shoemaker was an esteemed and celebrated astronomer and planetary geologist. Is it possible to localize their final resting place with a latitude, longitude and feature name?


texts: How to determine Az-El pointing of a telescope spiral scan?

texts:
I am writing a simulation of an automated telescope observation program.  One part of this is to run a spiral scan pattern on and around objects of interest.  I would like to tile this scan pattern so that each image is immediately adjacent to each other (perhaps with a specified level of overlap).  The telescopes I'm simulating are Az-El.

My primary objects of interest are GEO satellites, and that combined with the specific simulation framework used makes Az-El a little more natural to work with than celestial coordinates.

Given the Az-El of the object of interest, I'm trying to determine the Az-El pointing directions for all of the images.  As I understand it, I can define an image location in pixel coordinates relative to the central image centered on the object of interest and then project this into Az-El (or other spherical) coordinates.  I'm not entirely sure how to do this.  I see online and in codes lots of spherical projections (gnomonic, arc, orthographic, for example) but I don't know which one is the right one to use for what I'm trying to do.  Is my understanding of the procedure correct, and which projection should I use?


texts:
I was wondering if and how astronomers have/had detected the molecule Tetrahydrocannabinol (THC), commonly known for it's effects in marijuana, in space.

I had previously seen a hoax regarding the drug being detected on an exoplanet, but I wanted to know if there was any possibility of it.


texts: To find the mass and radii of hill sphere of a planet after passing through a region of planetary nebula

texts:
Consider a planet with initial mass m* kg and radii of hill sphere as r*.

The  planet it traveling (assume) perpendicular to the nebula at a speed of 13km/s and the nebula is approaching the planet at 15 km/s.

The nebula has a density of D kg/km^3.

The planet travels through this region for 1.4yrs in which it covers 570.5 million km.

Is is possible to calculate the mass and volume of hill sphere of the planet at the end of 1.4 yrs? If so how?
The problem I face is that with gain in mass the planets radii of hill sphere increases, hence more mass falls in which in turn again causes the hills spheres radii to increase, this process goes on and on till the end . I'm struggling to imply integration!

(In the hill sphere formula you can consider a and M to be constants)

Thank you!


texts: Could a close passing star be captured by the Sun's gravity?

texts:
Earlier I read an article here stating that a binary pair of stars passed within 0.8 light years from the sun. That made we wonder why the stars weren't captured by the Sun's gravity since the sun's gravitational influence spans over 1 light year.

Could a passing star be captured by the Sun's gravity? if so how close would it have to be before it was captured?


texts: Why are most planetary orbits nearly circular

texts:
In our solar system, with the exception of Pluto all planets follow a relatively circular orbit around the Sun, at the same inclination. They also all rotate in the same direction, none are 'retrograde'.

An image of the orbits of our solar system.

How and why have they all formed into these orbits? Is this reflected in most orbital systems? What factors affect the orbits of newly formed systems?


texts: Could undiscovered smaller black holes within galaxies be an explanation for dark matter?

texts: From what I've read, given the speed of stars rotating within a galaxy and the known mass, the stars' speed is too fast to stay in orbit. Dark matter was proposed to explain this but is it possible that there are undiscovered smaller black holes with no orbiting stars?


texts: Why do we see galaxies with their actual spiral shapes?

texts:
Since the actual position of stars is relative, and we see the light they emitted long time ago, and one galaxy has thousands of millions of stars, and those stars can be separated by hundred of thousands of light years, why don't we see galaxies as abstract forms or lines in the sky  as the light of every galaxy star arrives to us at different times instead of well formed spiral shapes? It's like if those millions stars were forming the galaxy as a single light spot, instead of millions of spots separated by light years.

This has been in my head for decades, but I didn't find the question (not even the answer, but the question) anywhere, so either there must be an really easy answer to it or a really complex one or it's a glitch in the Matrix.


texts: Earth rising and setting from moon's perspective

texts:
Why the earth is rising and setting seen from the moon,
when the moon is tidally locked?
Shouldn't the earth be always on the same spot because of the tidal lock, if observed from the moon?


texts:
Still learning, thinking - and failing - about the moon.
timeanddate.com has this cute and dynamic moon sketch at e.g. https://www.timeanddate.com/moon/uk/london

They generate this based on three parameters that they call i, p and r. The above image is the result of invoking this: https://www.timeanddate.com/scripts/moon.php?i=0.028&p=3.388&r=0.035.
I know that i stands for i llumination in the range 0-1.
However, what are p and r and in what unit? What are the likely correct terms? r appears to be some sort of "rotation angle" of the moon while p is the same for the "mask".


texts: Movement of the satellites of the planets . . .

texts: Is the movement of the satellites (moons) of a planet coplanar, like the planets being coplanar around the local Sun?


texts: The activity of the galactic nucleus is cyclic?

texts:
Are there observations/theoretical calculations that prove or disprove a cyclic activity of the galactic nucleus (I mean the nucleus is active than stops than become newly active with a time comparable to the galaxy rotation)?
I remember a lecture given by an astronomer two or three years ago in Arcetri that was speaking of a period of activity and inactivity of the Galaxy Nucleus. The speaker was referring to this phenomena as something that was emerging from observations, so this might very be a well known answered question but I couldn't find anywhere.


texts:
It is known as a fact that Earth's rotation is slowing every year, that's why we sometimes have 61 seconds long minutes.

In the Java documentation of Time : http://docs.oracle.com/javase/7/docs/api/java/util/Date.html , it is said :

  Some computer standards are defined in terms of Greenwich mean time
  (GMT), which is equivalent to universal time (UT). GMT is the "civil"
  name for the standard; UT is the "scientific" name for the same
  standard. The distinction between UTC and UT is that UTC is based on
  an atomic clock and UT is based on astronomical observations, which
  for all practical purposes is an invisibly fine hair to split. Because
  the earth's rotation is not uniform (it slows down and speeds up in
  complicated ways), UT does not always flow uniformly. Leap seconds are
  introduced as needed into UTC so as to keep UTC within 0.9 seconds of
  UT1, which is a version of UT with certain corrections applied. There
  are other time and date systems as well; for example, the time scale
  used by the satellite-based global positioning system (GPS) is
  synchronized to UTC but is not adjusted for leap seconds.

Why is that so? How can this occur?


texts: Would the Event Horizon Telescope be able to produce a superior image of Betelgeuse?

texts: Current images of Betelgeuse are already amazing, but I was wondering if the EHT could be able to make a significantly better image, given that Betelgeuse is pretty big and pretty bright?


texts: Why do radio telescopes, but not shorter wavelengths, have this Big Data problem?

texts:
Only with new radio telescope arrays like ALMA and SKA, have I heard of this "problem" with too much data come up. Too big to store. That one cannot archive it all for future studies, but that one has to select beforehand what fraction of the data to analyse and erase the rest, and the most, of it.

Shouldn't it be the other way around, that shorter wavelength telescopes like those in IR and the visible and UV generate more data because they have higher resolution per unit of aperture?


texts:
Cameras are great, they allow us to compress several seconds of light exposure into a single instant. This allows us to take amazing pictures of the night sky, like this one.

Of course you can see we also have a great view of the landscape, we know that this is a trick from a camera and we're collecting a timed sample of light. This isn't something a human eye can see because we can't compress time into a single vision.

I'm curious though -- people knew about the milky way long before we could do this cool trick with camera. That means they must have seen at least something of the concentration of stars and gases from earth -- So it makes sense that without light pollution this could be seen with the naked eye to some extent -- What does the milky way look like from earth to the human eye without light pollution?


texts: Why aren't neutron stars full of dark matter?

texts:
Dark matter interacts with the gravitational force right? Well, unlike black holes, neutron stars are actually visible, and they're an enormous gravitational sink, so dark matter should collect to them.

But if all that is true, which it seems to be, why haven't astronomers detected or used neutron stars to detect dark matter?


texts: Formation and capture of Mars' moons, Phobos and Deimos

texts:
Mars's moons have the appearance of captured asteroids.  How could this capture have taken place.  An asteroid in solar orbit would need to loose some momentum to be captured.  The gas Giants can capture new moons through 3 body interactions between the asteroid and their existing moons,  but Mars doesn't have this issue. Airbreaking would lead to impact, not capture.

An answer here suggests that the moons could have formed recently (millions of years, not billions) after an impact ejected a substantial amount of material in Martian orbit. What is the evidence for this?  Is there an impact crater of the right size, and age? Were both moons formed together?


texts: Finding galaxy spectra in SDSS Catalog by object name

texts: I am looking to find galaxy spectra of early type elliptical galaxies in the SDSS catalog by the objects name. See: https://dr12.sdss.org/advancedSearch. I dont know how to find out the PLATEID of my objects of interest, e.G. NGC 4125, NGC 4472 (Messier 49) or NGC 4649 (Messier 60), in the advanced search. I have tried to work with the constrains for redshift and sky position, but in the sdss library the objects name never turn up so I dont know if the found spectra are the spectra of the galaxies i mentioned. I am new to working with astrophysical data and the sdss catalogue in particular so please forgive me if this should be approached completely different. Can anyone tell me how to find these galaxy spectra fits files on the sdss website ? I am planning to further work with these fits files in python and extract kinematical parameters.


texts: Andromeda Galaxy is the largest galaxy in the galaxy cluster[1] called Local Group[2] and our Milky Way Galaxy as well as some satellite/dwarf galaxies are orbiting around each other within Local Group. My question isn't Milky Way Galaxy supposed to orbit around Andromeda Galaxy[3] so how come the predicted merger[4]? Whose slowing down or do the gravity work differently as things are being scaled further up?


texts: If gravity precedes the formation of a solar system, where did the mass come from that caused the gravity?

texts: In my class we are studying objects in our solar system and this question seemed to just pop up. And since I cannot answer this, I've been really frustrated for quite a while now and would like some help on understanding this.


texts: Is sundial time entirely dependent on solar azimuth?

texts:
I've visited several "how sundials work" sites and can't seem to get a
clear answer to this: is "sundial time" just a linear function of
solar azimuth? More specifically:

When the sun is due south (northern hemisphere), it is sundial
noon. All sites I've visited agree on this.
When the sun is due west (azimuth 270 degrees), I say the sundial
time is 6pm, a quarter turn/day from noon. However, I can't find a
site that actually says this, and some sites seem to disagree with
this. Same for it being 6am sundial time when the sun is due east.

I know there are different types of sundials, but had always assumed
they would give the same sundial time. Is that not true?

As a note, the sundial I describe above can be fairly inaccurate at
times, which makes me question whether I'm correct.

EDIT (to clarify question): Forgetting entirely about clock time for a
moment, suppose I build a sundial. When the sun is due west, my
sundial reads 6pm. When the sun is due east, my sundial reads
6am. When the sun is due south, my sundial reads noon. My question:
have I built my sundial correctly? The type of answer I'm hoping to
get:

No. When you build a sundial, it should read (something else) when
the sun is due west.
There are many different types of sundials. Depending on type,
your sundial may or may not read 6pm when the sun is due west.
It depends on your latitude: sundials are
latitude-specific. There's no such thing as a global sundial.
Yes, your understanding of a sundial is correct. Sundials can be
off by as much as (some number/formula) from mean solar time,
depending on your latitude. They can be as much as (some
number/formula) from clock time, depending on your longitude and
time zone.


texts:
How were interstellar reddening values for cepheid first calculated?

I mean, other than using the derived equation to get extinction-corrected magnitude of Cepheids, I want to know how to drive them independently from measured raw data (maybe V, B-band colors, etc).


texts: Python 2D Jean Instability in Spiral Galaxy Simulation

texts:
I have ~1000 snapshots of a spiral galaxy simulation (from Molecular cloud perturbations t=0 to t>1Gyr. What I need to do is determine whether any position at any of the snapshot times meets criteria for Jeans instability.

The simulation is 2D, and the snapshot script can read position, velocity and surface density ∑, but not temperature. The simulation also has no gas, instead the simulation was modeled by giving each individual star extra mass which corresponds to gas.
I looked online and several site state the Jeans mass in 2d equal to c$_{s}$$^4$ / G$^2$*∑. But I have no clue how to determine speed of sound c$_{s}$.
Also I am not really interested in Jeans mass, more so whether any position can collapse to make stars, i.e. satisfy Jeans instability.


texts: Are the Trappist-1 planets close enough to be tidally heated?

texts: According to the NASA press release, the Trappist planets are close enough (only a few million kilometers) that "the tidal forces between the planets are not negligible".  The speaker says that this could cause ocean tides on the planets.  Are they close enough that the tidal forces could heat the interior of the planets?


texts:
How much does the equivalent width of a line change by the introduction of 5% scattered light? We know the equivalent width is defined as $W = \int_{-\infty}^{\infty} \bigg(\frac{1-F_{\nu}}{F_c}\bigg) \, d\nu$; where $F_{\nu}$ represents the flux in the line and $F_c$ represents the flux in the continuum.
The measured equivalent width is $W_m = \int_{-\lambda_o}^{\lambda_o} \frac{I(\lambda)*(F_c - F_\nu)}{D_c} \, d\nu$ in which $\lambda_o$ is the spectral range over which the profile can be traced, $I(\lambda)$ is the instrumental profile, and $D_c$ is the apparent continuum.
If we choose $\lambda$ to be 0 at the center of the line, and the range spans 200 Angstroms, then does the equivalent width of the line change by $200 A \cdot 0.05$ = 10 A? So is the equivalent width of a line changed depend on our range? I.e. width of a line of 140 Angstroms with 5% scattered light would alter it by 0.7 A. Am I correct?


texts: Earth is part of Solar system, which is said to have originated from condensation and mutual attraction in dense clouds of gasses and dust. If Sun is basically more than ninty percent hydrogen and rest is helium. Nuclear fusion inside sun is said to pause near iron(56/26) and also several other planets like Jupiter and Saturn are gas giants. Where did heavy metals and other heavy elements on earth come from if second most nearest star(/star-system) is some four light years away (and mass would take long to come from those sources) ?


texts: Data Analysis: Image Sharpening through Posterisation

texts:
I had this image, a picture of Jupiter:

I wanted to measure the pixel diameter of Jupiter. As evident, the image is quite blurry, and thus, many value of equally justified pixel diameters can be obtained. To eliminate this I posterised the image (which is technically, quote from Gimp, "designed to intelligently weigh the pixel colors of the selection or active layer and reduce the number of colors while maintaining a semblance of the original image characteristics"):

With the above result, I obtained three pixel diameters of Jupiter (one from each of the coloured circles), and with a little bit of calculation, realised that the yellow circle gave rise to the most accurate value of Jupiter's diameter (which was surprisingly off by less than one percent!).

My questions are:

1) Why is my original image like that? Is it due to some form of light scattering?

2) Is there a better method of obtaining an accurate pixel diameter than posterising the image

3) Is the posterised result expected? Are there names that can be given to the three distinct spherical objects (which seem to resemble shadows, like the umbra, prenunbra, antumbra, etc.)? Is there a reason why the middle posterised object gives the most accurate value?


texts:
What do falsifiability and verifiability actually mean as a precondition of a scientific hypothesis?
I have been hearing from science guys that unlike philosophical hypothesis or theory, to be a scientific hypothesis an educated guess must have to be falsifiable and verifiable. But I cannot comprehend falsifiable and verifiable conditions for some scientific hypotheses especially in astronomy field. For example,
Couple of scientific hypotheses exists about the shape of the universe,

Universe is positively curved
Universe is negatively curved
Universe is flat

These are the well known scientific hypotheses as far as my knowledge (correct me if I am wrong). It seems to me, though I may be wrong, none of the above hypotheses are anyway falsifiable and verifiable because it is never possible for us to measure the geometry as the universe is expanding beyond our scope of study. For example, we can may be able to test on larger amount space as time goes on and technology advances, but to measure the actual shape of the whole universe we need to know theoretically the full picture of the universe which is not possible for us according to current understandings of science.
Now, there might be many such hypotheses which we cannot take under testable and verifiable condition yet we call them scientific hypothesis. My question is,

Is a scientific hypothesis really required to be falsifiable and verifiable? If so, what are the falsifiable and verifiable conditions for above mentioned hypotheses?


texts: Are the Voyager spacecraft outside of our Solar System?

texts: The wikipedia says that Voyager 1 cross the heliosphere, but is really outside the Solar System, or need to cross the Oort cloud?


texts: Is there a set of standard abbreviations for the days of the week for use in astronomy? I couldn't find any reference to it in the The IAU Style Manual (1989).


texts: After hearing that it has been confirmed that we found seven similar exoplanets like earth. I thought exoplanets were dead stars. After doing a little bit of research I found out that exoplanets are planets that are orbiting a star, NOT the sun. But isn't a star like the Sun? Can someone please explain this to me?


texts: Scattered Intensity of Light vs Angles of Incidence and Emission

texts:
So I just derived an expression for the scattered intensity of light from an atmosphere with ideal isotopic scattering: $I/F=\frac{cos(i)}{4(cos(e)+cos(i))}$, where $I$ is intensity, $F$ is flux, $i$ is the angle off incidence, and $e$ is the angle of emission ($i,\ e=0$ are normal to atmosphere).

When I plot the scattered intensity as a function of $i$ with $e$ constant, it decreases while the angle increases. Why is this? My theory is that at a large incidence angle, the light encounters a larger surface area, so reflected light will have a higher variation of emission angle, meaning that scattered light at a given emission angle will be lower. Does this make sense? Or is there another reason for this?

Next, I plotted scattered intensity as a function of $e$ with $i$ constant. This time, the intensity increases with increases emission angle. I don't have a theory as to why this is the case, nor an intuition to back it up. Why would this be the case?


texts: What happens when a star gets hit by a supernova?

texts: This may sound like a strange question, but say that two stars are relatively next to one another in a binary star system, what would happen to one of the stars if the other went supernova? Would it explode as well, or would it survive the blast?


texts: What is equation of straight line for a ideal blackbody on the color color diagram?

texts:
The color-color diagram is a plot between $U-B$ vs $B-V$ values.

For a ideal blackbody, what is the equation of this line?

In other words, what does the linear relation between $U-B$ and $B-V$ look like?


texts: What is the distribution of dark energy?

texts:
Dark matter is not distributed evenly. It forms clouds, it concentrates in galaxies, it forms hairs around celestial bodies.

A google search for dark energy distribution floods me with dark matter distribution results.

How is dark energy distributed? Does it cover the space uniformly, at even density, or does it create areas of concentration or other irregular forms?


texts:
Edit: just found duplicate

Since orbital velocity is greater on small orbits, and smaller on large orbits, why don't planets spin retrograde around their own axis?

One portion of planet's mass is closer to its star, and should move prograde relative to center of mass.

Part of the planet's mass far from the star orbits higher and should move retrograde relative to center of mass,

which should create a resulting retrograde spin, with rotation speed depending on planet's diameter.

What's wrong with this? ( since fact is : most planets spin prograde )

In this not to scale second illustration, planet is replaced by two masses linked by a beam. Kepler's law of equal surfaces swept per unit of time is used
to display the retrograde rotational force that should be applied relative to center of mass, which orbits black path.

EDIT : Ok I think I found where my mistake is(?) Differential gravitational pull (yellow arrows) counteracts retrograde rotational force (green arrow) Which results in no rotational force at all. Anyway I still don't get how conservation of momentum of everything orbiting the sun, also applies to spin direction of orbiting objects.


texts: What aliphatic compounds were found on Ceres, and how where they identified?

texts:
Los Angeles Times's article NASA's Dawn mission finds life's building blocks on dwarf planet Ceres says;

  While the scientists aren’t sure exactly what the compounds are, the fingerprint is characteristic of material containing carbon-hydrogen bonds, and may include components like methyl and methylene.

I've located the mentioned Science article Localized aliphatic organic material on the surface of Ceres but it is paywalled.

What was found? Is it some spectroscopic feature? Aliphatic compounds are hydrocarbons that are not aromatic. Is there a spectral feature that says "I am not aromatic" specifically, or does this just mean that aromaticity was not detected?


texts: Is there a maximum size for a black hole?

texts:
As far as I understand it black holes radiate Hawking radiation and probably gravity waves, which over time causes them to lose mass and eventually evaporate after almost unfathomable amounts of time.  I've also read that black holes eventually have difficulty gaining size after a certain point because gravitational forces working in a black hole's accreditation disk can eventually begin ejecting in-falling matter before it can be absorbed.

While I know black holes can become quite massive, that leads me to wonder if Black holes have a maximum possible size where they will begin to immediately radiate any additional mass that's added to it.  Or can a black hole effectively expand forever as long as the supply of matter is greater than the effects of radiation and ejection?

Is there effectively a size limit to black holes?


texts: How long should it take for us to observationally determine if Caltech's Planet Nine is really there?

texts:
Caltech just released a report that says there is possibly a large (10 earth mass) planet in a remote orbit (10 - 20 thousand year) that explains a lot of observations of Kuiper Belt objects. How long could it take for this to be confirmed or refuted observationally?

See this video


texts:
I've read many interesting threads concerning the composition of different planet cores in the solar system while trying to see if this question has already been asked. It appears as if Earth is unique regarding magnetic field strength and interior activity for our size compared with our rocky neighbors.

I was wondering if the general consensus was that most rocky-type planets our size would share the same characteristics (such as magnetic field strength and convection flow) as us? Or if we are the outliers and most extra solar planets similar to us are expected to be more like Mars or Venus?


texts: Pulsar signal periodicity detection algorithms?

texts: I am currently working on pulsar signal periodicity detection. Most of the algorithm or paper which i read they use FFT (Fast Fourier Transform) and FFA (Fast Folding Algorithm ). Do we have any other algorithm that we can use to find the periodicity of pulsar signal because pulsar signal is highly noisy so as per my knowledge the LSTM can't be used to find the periodicity of pulsar signal? So if we have any alternative method please suggest me.


texts: How could universe inflate itself out of the very dense and curved early spacetime? Could it happen in a black hole too?

texts: Wasn't spacetime as much curved as a black hole directly after Big Bang, because mass was so densly packed? Wasn't everything like an event horizon and how could things expand across it? Could inflation happen in a black hole too so that its content does become accessible to the outside universe?


texts: Do we know the exact spot where big bang took place?

texts: If you rewind the universe back 14,7 billion years, all matter were in one spot, and then started expanding. Do we know where this is in reference to our own solar system? And is there anything there? Or are all matter in the universe just blown inn all different directions and none is left at the "original" spot where it originated


texts: Why do we define the Milky Way's rotation in a left-hand coordinate system?

texts: When we talk about Milky Way kinematics, we define it as rotating clockwise from top.  In the rest of physics, rotating objects are defined as rotating counterclockwise from top (following the right-hand rule).  Why do we do the opposite when talking about the Milky Way?


texts: Why can't our Sun be a binary with Jupiter as a T or Y dwarf?

texts:
I just learned about Brown Dwarfs, they are "failed" stars, they narrowly missed the stellar mass mark. I learned that Y Dwarfs have temperature as low as 80 Fahrenheit (The first one found by WISE observatory), why aren't those not called a Jupiter-like planets without a host star? Why do we call them a star and designate a separate category for them (T or Y)?

Ok suppose T and Y dwarfs are okay, let's leave them alone. But why can't Jupiter be a Y Dwarf who is in the binary relationship with the Sun? Sun-Jupiter's Barycenter is just outside the Sun, we could call them in orbit with each other correct?


texts: Could the earth become a star if more mass was added?

texts:
I've heard that the only difference between a star and a planet is mass, meaning that if planets accredited enough mass they would too become stars.

Does this mean that the star started off as a planet?
Stars require hydrogen fusion and earth has little H. Could the earth become a star if more mass was added, but its relatively low abundance of hydrogen remained the same?


texts: If the Universe is Flat, why does it look 3D from our View?

texts:
So I know that the universe is flat in a 4D point of view, but what about a 3D point of view? The universe does not SEEM flat from our view, so how can it really be so? Please answer more simple, im not an astronomy pro.

Sorry if this is a dumb question, I don't have a degree in astrophysics or anything, i'm just a kid who is really interested in the universe, so i'm sorry (in advance) if this is a simply answered bad question.


texts:
A sentence from the abstract of The population of natural Earth satellites states:

  At any given time there should be at least one NES of 1-meter diameter orbiting the Earth. The average temporarily-captured orbiter (TCO; an object that makes at least one revolution around the Earth in a co-rotating coordinate system) completes (2.88 ± 0.82) rev around the Earth during a capture event that lasts (286± 18) d.

As far as I know, the only known, well documented temporarily captured Earth satellite is 2006 RH120, which was in within 1 million km of the Earth for about 1 year. It has an estimated size of 2 to 3 meters, a rotation period (about it's axis) of about 3 minutes (presumably determined by photometry). According to Wikipedia:

  2006 RH120 was discovered on 14 September 2006 by the 27-inch (690 mm) Schmidt camera of the Catalina Sky Survey in Arizona.

Using data from the JPL Horizons database, it seems 2006 RH120 was only about 850,000 km from Earth at the time it was discovered.

Deep sky surveys are extremely prolific at detecting small solar system bodies. There was also a recent answer addressing the possibility that the Gaia spacecraft's could detect trojan asteroids associated with Earth's orbit, linking to the paper Detection of inner Solar System Trojan Asteroids by Gaia.

The question asked specifically about TCO's being detected by Gaia, but my question is more general: How would a small TCO (temporarily captured orbiter) or other natural Earth satellite most likely be detected?

There are Earth-based sky surveys, space based visible and near-visible, and possibly even infrared/thermal detections possible, even Earth-based radar, and possibly others.

Are there any searches specifically targeting TCOs?

above: a simple plot of the orbit of 2006 RH120 from JPL Horizions transformed to an Earth-centered Earth-Sun-synodic frame - red line with dots every 10 days, for the period when it was within a +/- 1.6 million km cube centered on Earth in this frame. The green thick band around Earth is the moons orbit during this 448 day period.


texts: Natural units of star clusters angular momentum and energy

texts:
I'm calculating the total angular momentum of a bunch of theoretical stars of a few solar masses, separated by a few light years, moving at velocities of about a few dozens of km / sec.

Mechanical angular momentum relative to an arbitrary origin (the cluster's center of mass is a good choice) is this :
\begin{equation}
\vec{L} = \sum_i \vec{r}_i \times m_i \, \vec{v}_i,
\end{equation}
so the SI units are $\mathrm{kg} \cdot \mathrm{m}^2 / \mathrm{sec}$.  But in real astronomy, what should be the most simple/natural units for star clusters ?  $M_{\odot} \cdot (\mathrm{ly})^2 / \mathrm{year} \equiv M_{\odot} \, c^2 \cdot \mathrm{year}$ ?

What are the angular momentum units used by astronomers ?

And what about energy units for a cluster of stars ?  I'm interested in kinematical energy and gravitationnal potential energy of the whole cluster.  Certainly not joules !?  And $M_{\odot} \, c^2$ would certainly be too much, I suppose !


texts: How can the life time of a multiple star system, such as for example the trinary system PSR J0337+1715 be derived?

texts:
As for example explained at the beginning of this blog post, the trinary system consists of a millisecond pulsar ($1.438$ times the mass of the sun) orbited by two white dwarfs. One of the white dwarfs ( $0.198$ solar masses) is very close to the pulsar and has an orbit period of $1.6$ d, whereas the other one ($0.410$ solar masses) is farther away and and needs about a year ($327$ d) to orbit the central pulsar.

Such a three-body system is in principle expected to show a chaotic behavior sooner or later, which means that collisions between these three celestial bodies can be expected and a finit life time of the system can be assumed.

Making some in my opinion way too hand-waving arguments, the blog post further explains that the collisions can not be expected too soon however, by taking into account that the distant white dwarf "sees" the inner white dwarf and the pulsar as a single central body and the relative motion of the inner white dwarf around the pulsar is rather stable and eliptic too.

Thinking about such multiple star systems as chaotic dynamical systems, another approach to estimate the liftime could be to make use of some chaos-theoretic methods which could for example involve the Lyapunov Exponent of the system, such that a large exponent would mean that the collisions happen soon and the stellar system has a rather short life time, whereas the converse would be true if the Lyapunov exponent is small (which is what I would expect for the system in my question).

So in short my question is: how can the liftime of a multiple star system be calculated in a not just hand-waving way?

This question is interestingly related to my issue, but it does not yet answer it...


texts: Why are Delta Aquariids "for the southern hemisphere" while the Perseids are "for the north"?

texts:
In the video How to spot Comet NEOWISE, plus the largest 3D map of the Universe | Night Sky News July 20 after 05:28 Dr. Becky (Smethurst, Oxford astrophysicist) says:

…and it just so happens that we’ll be passing through the debris trail of two of the comets whilst comet NEOWISE is going to be visible in the sky, and so that’s going to cause to very popular meteor showers. There’s the Delta Aquariids meteor shower at the end of July, and then the Perseids meteor shower mid-August time.
And the Perseids meteor shower is just hands down the BEST meteor shower of the year. There’s something like sixty meteors per hour during its peak which this year is going to be the 12th of August.
Now the Delta Aquariidss are probably more like fifteen to twenty meteors per hour, so not quite as spectacular as the Perseids, but the Delta Aquariids really are more for the southern hemisphere where the Perseids is really going to be for the Northern hemisphere.

Question: Why are Delta Aquariids for the southern hemisphere while the Perseids are for the north? The Earth is moving along its orbit at 29 km/sec in both cases, why the big difference?
note: there are notes below the video that you might find helpful if looking for comet NEOWISE.


texts: What is meant by "short lived" in the duration of a planetary nebula?

texts:
In this wikipedia article, it describes planetary nebula as "short-lived," but I'm not sure in relation to what?  This could mean billions of years in some contexts.

How long is short-lived?

Bonus if there are any useful rules of thumb in astronomy terms for future uses (how stars timelines are described, versus galaxies, etc.)


texts:
Imagine an astronaut standing on an asteroid 2.5 A.U. distant from the Sun. He is gazing, naked-eye, towards the Sun.

What is the apparent magnitude of the Sun at that distance, and is that bright enough to blot out any of the inner planets? No light blockers and solely viewing with the Eyeball Mark I?


texts: Perception of an objects history passing as an observer moves toward it

texts: If a star is 20 light-years away and I look at it through a telescope, what I'm observing is what the star was doing 20 years ago yes? So, if I fix my sight on the star and move toward it (for example's sake, let's say 0.5C or half the speed of light in a vacuum) in a linear path (which is not practical for real space travel) it would take me 40 years to reach it. By the time I reach it I would be observing in (almost) real-time what is happening. So to me that says that I've watched 60 years of the star's history in 40 years worth of time. So then, would it look to me as if I was watching a movie of the star's history in 1.5 times 'normal' speed? (60/40 = 3/2 or 1 1/2)


texts: How to convert ra&dec between different epoches?

texts:
For example, how to convert B1950.0 into J2000.0?

Do we need to know the proper motion of the star and the time it was observed?


texts: Looking for a celestial body that weights ~$10^{17} kg$?

texts:
I am exploring the outcome of a problem assigned in a quantum mechanics homework where the mass of of some beam of "something" is insignificant to the result. So much so that something weighing approximately $1.2285 * 10^{17} kg$ would be needed to make a difference in respect to the problem. (that is, assuming I've solved the problem correctly).

I have been googling for quite a bit and I can't find anything that is in the $10^{17} kg$ range. Does anything come to mind?

(I can't seem to find any good tags for this question)..


texts: With the current news about the possibility of a planet in the Oort cloud, I was thinking, why is the matter in the Oort cloud distributed so? It makes sense that inside the planet region there is not a huge concentration of comets and meteors, but why is the matter inside the cloud itself not agglomerating? It seems like there is enough matter to form not one or two, but many more planets.


texts: How old is the oldest light visible from Earth?

texts:
Because light can only travel so fast, all of the light we see in the sky was emitted at a previous moment in time.  So if for example we see a supernova or some other great stellar event, by the time we see it, it maybe long over.  That made me kind of curious, what is the most ancient light we can see from earth?

The universe is supposedly ~13+ billion years old, but we are probably not at the very edge of the known universe so all the light we see is probably less than 13 billion years old.  So what is the oldest light we can see? and as an optional follow-up question how do we know the age of that light?

I guess the light itself may not actually be literally 'old', but its probably obvious what I'm asking here, put another way: what's the longest distance that now earth visible light emitted has traveled to reach the earth?  Though that reformation of the question gets kind of tangled with lensing effects.


texts: How long does it take for a white dwarf to cool to a black dwarf?

texts:
I was reading on white dwarfs, and I came across this sentence—

  Without energy sources, the white dwarf cools to a black dwarf in a few billion years.[1]

However, when I looked into the Wikipedia page on White dwarf, it says

  Because the length of time it takes for a white dwarf to reach this state is calculated to be longer than the current age of the universe (approximately 13.8 billion years), it is thought that no black dwarfs yet exist.

So which is true?

And what is the proper definition of a black dwarf?

References:

[1] Introductory Astronomy and Astrophysics. Zellik, M. Gregory, S. 4th edition. Brooks/Cole. 1998


texts:
I have two ephemerides for orbital elements—that is, each contains $a$ (semi-major axis), $e$ (ecc.), $i$ (incl.), $\Omega$ (long. asc. node), $\omega$ (arg. of peri.), $\tau$ (time of peri.), $T$ (period), and a timestamp $t$.

I want to compute a new ephemeris at a point somewhere between these two ephemerides.  For example, the timestamp ($t$) of the new ephemeris can be calculated by a linear interpolation of the start ($t_0$) and end ($t_1$) ephemerides' times as:$$
    t = \text{lerp}(t_0,t_1, x) = t_0 \cdot (1-x) + t_1 \cdot x
$$

Unfortunately, I can't seem to get an interpolation for the other parameters that doesn't have discontinuities in it.  My question: how should I be interpolating ephemerides?  Maybe I need to do this through some intermediate parameters, like $\varpi$, $\lambda$, or $\nu$?

  For example, if I take $\tau$ from the first ephemeris and lerp everything else, I get a strong discontinuity when mean anomaly is calculated (as $M:=n (t-\tau)=(2 \pi/T) (t - \tau)$).  I suspect this is because the mean anomaly calculation is only valid at the ephemeris, and as e.g. $T$ changes due to interpolation, $M$ changes erroneously.

  Here is an example for Mercury.  The first three ephemerides come from NASA HORIZONS and are correct.  Interpolating the first two ephemerides produces $M=328^\circ$.  However, two seconds later, the interpolation (which is now interpolating from the last two ephemerides) produces $M=294^\circ$.


texts: Would space-faring vehicles in interstellar space be in pitch darkness, show reflections from the nearest star or be bathed in light completely?

texts:
This is not a science fiction question, but I was considering the visual accuracy of ships in space depcicted in Battlestar Galactica and the like. They are always strongly lit.

If they are in between stars, would they be visible at all or bathed in light equally on all sides?

Pursuing it further, at what distance from a star does an object no longer reflect it's light as the dominant light source and after that is it dark?


texts: Why isn't Earth tidally locked to the sun?

texts: Given that the moon has long been tidally locked with the Earth, why isn't Earth (or any of our other solar system's planets) tidally locked to the sun?


texts: Does matter absorb/scatter gravitational waves?

texts:
When photons pass through matter, they do tend to get absorbed by that matter to some degree depending on the properties of the photon and and the matter. Even the brightest light is at least somewhat obscured by matter in its way.
Similarly, while neutrinos interact very little with normal matter, they are eventually absorbed as well: you just would've need a whole lot of matter for that to be efficient (such as one light year of lead to capture 50% of neutrinos)
Does matter have an effect on gravitational waves and their propagation, or do they just pass through the matter unaffected? If there is no effect, where does the energy needed to actually move things around come from (we can detect them due to this movement!)? If matter can block gravitational waves, how much matter we would need to block GWs from typical BH merger detected by LIGO / VIRGO (say 30 + 30 solar masses)?


texts: How long was a lunar month in ancient times?

texts:
As the Moon is getting farther and farther from Earth, it's natural that the time it takes to circle around our planet is going to increase, thus the month will be longer.
However, I read that the reason it's straying from us also is causing the Earth to revolve more slowly. As the day is getting longer, maybe a lunar month will remain more or less the same?
Fast rewind it back to billions of years ago, did that hold true too? How many 'days' did it take for the Moon to go around the Earth, say, 4bya? Are there any formulae to calculate this?
I'd appreciate more the answers that consider both the day length of specific ancient times and its conversion to modern day, for easy imagination. Any sources that list several values for different periods in the past would be great, too.


texts: How to view the surface of Mars with a 130 mm Newtonian?

texts:
I bought a Celestron AstroMaster 130EQ telescope, but I am not able to see the surface of Mars through using the 10 mm eyepiece. Instead I only see a glowing dot.
What should I do to view the mars surface?


texts: How does a supernova occur when a massive star collapses into a black hole?

texts: My layman's understanding of a supernova explosion is that it's caused by a rebound when a massive star collapses. When the core collapses to a point where neutron degeneracy pressure dominates, the core collapse stops, and the matter falling in from the outer layers bounces back, causing a supernova (please correct me if I'm wrong). However, I've also read that the end result of the supernova can either be a neutron star or a black hole depending on the mass. When the star is massive enough to collapse into a black hole, what causes the supernova? What do the outer layers bounce off of? Shouldn't the whole thing just collapse into the black hole?


texts: "LinAlgError: SVD did not converge in Linear Least Squares" with eleanor.py

texts:
I'm doing some tests with eleanor module (i'm a novice). I'm using the example code written on eleanor.py website
from IPython.display import Image
import warnings
warnings.filterwarnings('ignore')

import eleanor
import numpy as np
import matplotlib.pyplot as plt
from astropy.coordinates import SkyCoord

eleanor.Update(sector=1)

star=eleanor.Source(tic=38846515, sector=1)
print ('-------------------------------------')
print ('-------------------------------------')
print ('-------------------------------------')
print ('')
print ('')
print ('Found TIC {0} (Gaia {1}), with TESS magnitude {2}, RA {3}, and Dec {4}'
.format(star.tic, star.gaia, star.tess_mag, star.coords[0], star.coords[1]))

data = eleanor.TargetData(star, height=15, width=15, bkg_size=30, do_psf=True, do_pca=True, regressors='corner')

plt.figure(figsize=(15,5))

q = data.quality == 0

plt.plot(data.time[q], data.raw_flux[q]/np.nanmedian(data.raw_flux[q])+0.06, 'k')
plt.plot(data.time[q], data.corr_flux[q]/np.nanmedian(data.corr_flux[q]) + 0.03, 'r')
plt.plot(data.time[q], data.pca_flux[q]/np.nanmedian(data.pca_flux[q]), 'g')
plt.plot(data.time[q], data.psf_flux[q]/np.nanmedian(data.psf_flux[q]) - 0.02, 'b')
plt.ylabel('Normalized Flux')
plt.xlabel('Time [BJD - 2457000]')
plt.title('WASP-100')
plt.show()

After running the program, it find the star with its magnitude and coordinates, but appears this error:
LinAlgError Traceback (most recent call last)
c:\Users\gabri\OneDrive - Università degli Studi di Padova\Desktop\eleanor prova.py in
23
24
---> 25 data = eleanor.TargetData(star, height=15, width=15, bkg_size=30, do_psf=True, do_pca=True, regressors='corner')
26
27 plt.figure(figsize=(15,5))

C:\Python36\lib\site-packages\eleanor\targetdata.py in init(self, source, height, width, save_postcard, do_pca, do_psf, bkg_size, aperture_mode, cal_cadences, try_load, regressors, language)
227 self.create_apertures(self.tpf.shape[1], self.tpf.shape[2])
228
--> 229 self.get_lightcurve()
230
231 if do_pca == True:

C:\Python36\lib\site-packages\eleanor\targetdata.py in get_lightcurve(self, aperture)
623 norm = np.nansum(self.all_apertures[a], axis=1)
624 all_corr_lc_pc_sub[a] = self.corrected_flux(flux=all_raw_lc_pc_sub[a]/np.nanmedian(all_raw_lc_pc_sub[a]),
--> 625 bkg=self.flux_bkg[:, None] * norm)
626 all_corr_lc_tpf_sub[a]= self.corrected_flux(flux=all_raw_lc_tpf_sub[a]/np.nanmedian(all_raw_lc_tpf_sub[a]),
627 bkg=self.tpf_flux_bkg[:, None] * norm)

C:\Python36\lib\site-packages\eleanor\targetdata.py in corrected_flux(self, flux, skip, modes, pca, bkg, regressors)
1235 f = np.arange(0, brk, 1); s = np.arange(brk, len(self.time), 1)
1236
-> 1237 lc_pred = calc_corr(f, cx, cy, skip)
1238 corr_f = flux[f]-lc_pred + med
1239

C:\Python36\lib\site-packages\eleanor\targetdata.py in calc_corr(mask, cx, cy, skip)
1175 # temp_lc = lightcurve.LightCurve(t, flux).flatten()
1176 tmp_flux = np.copy(flux[np.isfinite(flux)], order="C")
-> 1177 tmp_flux[:] /= savgol_filter(tmp_flux, 101, 2)
1178 SC = sigma_clip(tmp_flux, sigma_upper=3.5, sigma_lower=3.5)
1179

C:\Python36\lib\site-packages\scipy\signal_savitzky_golay.py in savgol_filter(x, window_length, polyorder, deriv, delta, axis, mode, cval)
333 x = x.astype(np.float64)
334
--> 335 coeffs = savgol_coeffs(window_length, polyorder, deriv=deriv, delta=delta)
336
337 if mode == "interp":

C:\Python36\lib\site-packages\scipy\signal_savitzky_golay.py in savgol_coeffs(window_length, polyorder, deriv, delta, pos, use)
135
136 # Find the least-squares solution of A*c = y
--> 137 coeffs, _, _, _ = lstsq(A, y)
138
139 return coeffs

C:\Python36\lib\site-packages\scipy\linalg\basic.py in lstsq(a, b, cond, overwrite_a, overwrite_b, check_finite, lapack_driver)

LinAlgError: SVD did not converge in Linear Least Squares

I tried to find solutions, like installing numpy and scipy from conda, but they did not solve the problem. Anyone know how to deal with it?
EDIT:
I solve the issue by creating a conda environment, but if I run the program with py IDLE it crashes anyway.


texts:
This question relates to a diffuse hot gas halo of our Milky Galaxy. I've read that there is a hot diffuse halo of gas surrounding our Galaxy (NED, Caltech)

I was wondering why such a halo can exist? Why doesn't it collapse to a disk shape? Is it because the gas itself is still hot and so remains largely unaffected by the Galaxy potential?


texts: What is the Milne-Eddington Approximation?

texts:
In this paper: Planet temperatures with surface cooling parameterized it states in the "radiation model" the following:

The Eddington–Milne approximation relates $T_0$ and $T_e$ through the
gray IR optical depth $\tau$:
$T_0=T_e(1+\frac{3}{4}\tau)^{0.25}$

I looked this approximation up on google, and found it was more commonly known as the Milne-Eddington Approximation. However, this approximation seems to (mainly) be applied to stars, and I can't exactly see the connection between it and the temperature of a planet. (I also don't completely understand what an "optical depth" is.)
What is this approximation and why can it be applied to approximate the temperature of a planet with an atmosphere?


texts: Pluto's orbit overlaps Neptune's, does this mean Pluto will hit Neptune sometime?

texts:
We know that the orbits of Pluto and Neptune overlap.
This means that pluto sometimes crosses the orbit of Neptune; will Pluto hit Neptune in any circumstance?


texts: Retrograde motion of planets: reason and time?

texts:
As beginner, I would like to understand how planets exhibit retrograde motion.
When does the retrograde motion start for a planet like Mars and how long (time limit) planets will appear to be in such motion?
Any good URL or simple explanation will be helpful.


texts: In astronomical interferometry, what values do the points in the uv-plane have?

texts:
As I understand it, the image of an interferometer is the inverse fourier transform of the information in the uv plane. For each baseline (vector between any two telescopes in the array), representing a certain wavelength along its orientation, there is a point in the uv plane.

(image from lecture notes from nrado.edu)
But what value does each of these data points have? I'd expect them to contain amplitude and phase information, but which exactly?
How do I get these values from the signals of my telescopes? Say, I already computed the phase shifts between the signals, what values do I put in the uv plane?


texts:
If i know the radius r of a star in cm, how can I convert it to arcseconds?

For example, if I have a star with r = 3.18e13 cm, and distance to the star d = 220 parsecs, what is the relation to convert the radius from cm to arcseconds.

Thank you.


texts: What's the influence of a tilted orbital plane, when observing an exoplanet transit?

texts:
I made an illustration to explain what i mean : If we assume we have two similar planet/star systems (similars in size/mass/period..) but tilted differently relative to us.
How can we predict that differences in dimming measurements (time & intensity) are caused by the tilting of orbital plane? (and not by the diameter of the exoplanet)


texts: How long does it take for a white dwarf to undergo a type Ia supernova?

texts: I read about white dwarfs and their relation to Type Ia supernovae, and I want to know how much time passes between when the white dwarf passes the Chandrasekhar limit and when the white dwarf explodes. So is there an exact value of this time, or does it vary between these events?


texts:
Using the centripetal force to create artificial gravity has been proposed for space exploration. From our perspective, the force is an outward force, pushing objects away. Could it be inverted to attract them instead?

Could we, instead of having a spacecraft, or a "dish," rotate to create the force that would push the travelers to the inside of the hull - could we instead have a cluster of dishes rotating in such a manner that the travelers are pressed to the outside of a sphere?

Or, could we construct an artificial mini-planet with a gravity exceeding by far the gravity of a natural celestial body with the same mass?


texts: Converting the log (Stellar Luminosity/Solar Luminosity) and the log of the plus and minus variation

texts:
There are some stars in the NASA Exoplanet Archive that I am interested in but I am confused by the physical meaning of the terms.

For example, there's a star BD+15 2940 with a log10 of Solar Luminosity of 2.01 +/-0.75 where the variation is also a log.

So is the linear equivalents 18.2 to 575.44 with most likely value 102.33?

I went to the citing paper Nowak et al. 2013and they quote it as log L*/LSun 2.01 +/- 0.75 as per the NASA archive.


texts: How dependent is Earth is with the other Planets' gravity?

texts: How would the Earth be affected if the Moon was destroyed as far as our orbit around the Sun? What if Mars or Jupiter was destroyed?


texts: How does an eclipse differ from an occultation?

texts:
A comment in response to this question suggests that an eclipse differs from an occultation in that the former casts a shadow while the latter doesn't.

This isn't particularly satisfactory since blocking the light from one object with a second will always cast a shadow even if it's not visible to the human eye.

Similarly, an annular eclipse of the sun by the moon could be referred to as a transit.

Is there a more rigorous definition of these three terms that distinguishes an eclipse from an occultation or a transit?


texts:
This is a somewhat simplified, and drawn / formulated in a manner more targeted to astronomy, version of a diagram and a question that I've also posted in math.stackexchange.com.

In the diagram below I depict:

the celestial sphere with the North Celestial Pole (NCP), the South Celestial Pole (SCP) and the celestial equator
a latitude circle at latitude $\delta$ (declination)
two points $A$ and $B$ on that latitude circle
the center of the celestial sphere $O$ and the center of the latitude circle $O'$

My question is:

Given angle $\phi$ between the rays $OA$ and $OB$, what is the angle $\theta$ between the rays $O'A$ and $O'B$ ? I am trying to derive $\theta$ given $\phi$ and $\delta$. Observe that if $A$ is the first point of Aries, then the angle $\theta$ is the Right Ascension of a star located at point $B$.

My assumption is that the following formula holds:

$\phi = \theta*cos(\delta)$


texts: What makes a really good observatory site, besides altitude?

texts:
Let's limit this to optical telescopes.

I understand that the higher you are, the less atmosphere is above you to get in the way of observations. From reading around, there seem to be a few more but I'm not sure:

Not near an active or dormant volcano (but Mauna Kea seems to disprove this?). This is kinda a bummer because a lot of tall mountains seem to be volcanic.
Somewhere with clear and/or dry weather for as much of the year as possible
Not near major light pollution like cities
Cold weather is better than hot weather? Not sure I understand that; if it's uniformly hot or cold then I don't see how it makes a difference.

Am I missing any? Or are these wrong in some way?

As a concrete example, would the Tibetian plateau make a really good observatory site? (Not considering construction or transportation costs to a remote area, of course.)


texts: What is the difference between the way light travels in a gravitational field and the way it travels through an expanding universe?

texts:
A photon is travelling through space. As it travels, the space through which it travels is expanding

Does this describe

A - a photon travelling from a distant galaxy to earth
B - a photon travelling from a point of higher to a point of lower
gravity
C - either

If like me you think the answer is C, can you explain why light should travel differently through a gravitational field to how it travels through an expanding universe? In other words, why in one case does the speed of the photon vary, while in the other case it doesn't?

Alternatively can you explain why you don't think it's C?

I have asked a similar question on the Physics Stack, but I probably worded it badly, so I just got a load of down votes and no answers. I hope I haven't offended anyone here with this wording.


texts: What is Earth's average temperature as a whole?

texts:
I would like to know, what is the average temperature of Earth as a whole planet, not just its surface temperature. Hope the following scenario will make it more clear what I mean:

Imagine, that we isolated/sealed Earth (up to 30 km above sea level) as it is right now, so that there is no mass or energy being added or escaping and waited several millions of years till the temperature is everywhere the same.

What would that temperature be? When I tried to google it, I only could find references to surface temperature, and not to this.

EDIT: Based on remarks in comment section:

I am not an expert, that is why asked this question, however IMHO of a layman in this area, I presumed that radioactive decay wont play that much of a role (like a +-1%). And the role of a gravity is well beyond me. But the most important thing is, I pretty much wanted to know the average temperature by heat capacity of the Earth at this moment (so basically, all decays and all tidal forces excluded, even if they play significant role).

I would just like to have (a fairly confident) idea about the potential temperature, and no need for highly exact numbers and calculations, +-15% is absolutely fine.


texts: Orbital elements of solar system using Pyephem

texts:
I'm using the astrometric python module Pyephem, and I would like to get the orbital (keplerian) elements for the  solar system planets.

The only values I found is the heliocentric latitude, longitude, and the distance to the sun. Is there a way to compte the orbital parameters based on thoses values ? Did I missed a function in Pyephem ?


texts: Now that LIGO has finally measured gravitational waves using a huge laser interferometer, to me, the question remains, why was it possible? As it is explained in many news articles, gravitational waves are similar to water waves or electromagnetic waves, they just do not exist in a medium like water or space, but space-time itself is the transport medium. If space-time itself gets contracted and expanded by the gravitational waves, so does any means of measurement. The ruler you use for measurement (the laser beam) gets deformed while the wave travels through the measuring device. Otherwise the "ruler" had to live outside of space-time, but there is no outside. If space-time was a cup filled with pudding, on which we had painted a straight line with 10 marks, pushing into the pudding slightly with our thumb does bend the line, but for us, there remain 10 marks on the line, because to measure the extension, we had to use a ruler, outside of our space-time (pudding) to measure, let's say, 11 marks. But, well, there is no outside. I assume the same happens not only to the 3 spacial dimensions but also to the time dimension. Because they "did it", what am I missing?


texts: Converting orbital elements from equatorial reference frame to ecliptic

texts:
I have a bunch of orbital elements (inclination, longitude of the ascending node, mean anomaly, argument of periapsis) that are all referenced from an equatorial reference frame. But the program I am trying to use requires the orbital elements in an ecliptic frame. Is there an easy/convenient way to convert between the two frames?
I also have the RA and Dec for the orbit pole, also in the equatorial frame. I am aware of some rotation matrices that allow me to convert between equatorial and ecliptic frames—should I just apply the matrix to the RA and Dec of the orbit pole? Is there a way to turn that into orbital elements?
Or should I turn the equatorial elements into a state vector (Cartesian position and velocity) and then apply the matrix to that, and then convert the ecliptic vector into orbital elements?
What is the easiest way to go about this?


texts: What is the probability of seeing a neutron star as a pulsar?

texts:
Pulsars are neutron stars that emit a beam of electromagnetic radiation that is not aligned with its rotation axis. If the Earth passes through that beam of radiation, we see a pulsar. Pulsars are only observable if the beam crosses the observer's line of sight. Otherwise, we can only see a regular neutron star.

For a pulsar with random orientations, what is the probability of seeing its beam from Earth? Out of 100 pulsars, how many will have a beam that crosses the Earth?


texts: If Venus and Mars changed places, would we then have 3 habitable planets?

texts:
This is the supposition:
Venus is too hot, Mars is too cold. If they switched places then Solar energy would change to make both more Earth like. We might as well have had three instead of just one "Earth" in the Solar system, if the roulette of early planet formation had played out just a little bit different.

That's of course too simple. But wouldn't it help alot if it were the case from the beginning that Venus had had Mars' mass, and Mars had had Venus' mass? Or do other factors dominate mass and distance from the Sun?

Would Venus still have had a thick, but not too thick, atmosphere if it were at 1.5 AU, because it has mass enough to keep one, and wouldn't that have kept it warmer than Mars is today? Including flowing water on its surface under its atmospheric pressure?

Would a Mars at 0.7 AU have been warmer and maybe have had a passing atmosphere created from melting volatiles during a longer era in its history?


texts: Compton scattering of high energy radiation

texts:
I was reading up on compton radiation, and a question came up which I could not quite answer.

Compton scattering is known to be less efficient at high energies, due to the fact that the Klein-Nishina cross-section falls off at the high energy end. This means that the Universe is more transparent to Compton radiation at high energies.

What other processes could stop high energy photons from propogating through the Universe (i.e., a process that preferentially stops photons at the high-energy end)?


texts: Do most astronomers think that Andromeda will collide with Milky Way?

texts: Most astronomers say it is likely to happen. Is this true? Are there any scientists that reject this?


texts: How much overlap will the Andromeda galaxy and the Milky Way have when they collide?

texts:
Measurements of Andromeda's blue shift let us conclude that the distance between the Andromeda galaxy and the Milky Way is decreasing and in a few billion years they will "collide".

The blue shift only yields the radial component of Andromeda's velocity vector. It is my understanding that measuring the tangential component is crucial in determining whether a "collision" will actually happen (in a gravitationally bound two body system, for point-like bodies to collide, the relative velocity component must point exactly towards the other body, i.e. the tangential component must vanish).

Now, galaxies are not point-like, so some small nonzero tangential component might lead to a collision where at least some galaxy arms intersect.

Has the tangential velocity been measured? If so, how? How central is the collision (bulge into bulge, bulge into arms, arms into arms)?


texts: What is CMB radiation doing to the universe?

texts: It is said that due to CMB radiation our universe has been expanding? Firstly what lead the universe to expand? What do scientists mean by dark energy? Will energy convert into matter?


texts: Where was this photo of Nancy Grace Roman taken, what is the display shown?

texts:
Where was this photo of Nancy Grace Roman taken, what is the display shown, is there any technical information on this display and where is it now? If this is a NASA photo then there should be an original ID or catalog number and hopefully explanation, but it may not be a NASA photo.
I spotted this in Nancy Roman Telescope’s Primary 2.4-Meter Mirror is Ready when looking for background information related to the question What “improvements in technology” allowed the primary mirror of RST (WFIRST) to be less than 1/4 the weight of Hubble's?

Source which links to this higher resolution version

See also

Hubblecast 113a
YouTube: Hubblecast 113: Nancy Roman — The mother of Hubble

The display is shown when the narration says:

Nancy followed her dream and became a professional astronomer, working in the new discipline of radio astronomy.  And in 1961 she became the first chief of astronomy in the office of Space Science at NASA.

I'm guessing that this display is for the position and other information for a radio telescope, but if so, which one?


texts: What are the nearest galaxies I can observe?

texts: What are the nearest galaxies I can observe using my Telescope? Does it require to be out of the city lights?


texts: How accurate should sun\moon ephemeris be for calculating eclipses?

texts:
I am writing a program to simulate eclipses for about -2000 to 6000. My question is, how accurate does my ephemeris for the sun and moon have to be to get a decent accuracy for eclipses? I currently have an accuracy of about 1'' for the Sun and an accuracy of about 12'' for the moon. Is this good enough, or should I find a more accurate system?

Edit:
I'm still rather new to calculating eclipses, though I have some experience with calculating ephemerises. I'd like to calculate several things: when they occur, the duration, the path of the shadow on the earth for a solar eclipse, and perhaps the percent covered. I suppose I'd like the accuracy on these to be about $\pm 0.005$ or better. I have access to several resources that should help me do this, but I need to know how accurate to get the ephemeris.


texts: Jupiter/Saturn's fate after Sun goes red giant and planetary nebula?

texts:
The solar wind contributes a very small amount of matter to the gas giants.  As the sun approaches the red giant phase, the solar wind increases significantly.  10% of the sun's mass lost as solar wind over its lifetime, which is equal to 100 Jupiters in mass.  After the red giant phase the sun releases the outer, mostly hydrogen, outer layer in the form of the planetary nebula. The planetary nebula is about 50% of the mass of the sun, which is equal to 500 Jupiters in mass, the remaining 40% of the sun will be left as the white dwarf.

After all this additional mass, most of which is hydrogen, is added to the gas giants how massive would they be and would any of them reach brown or red dwarf status?

Smallest brown dwarf is 13 Jupiters in mass, which is able to fuse deuterium. Smallest red dwarf is at 75 Jupiters in mass, which can fuse hydrogen.